refactor: collapse fakewallet/regtest wrappers into single dev CLI

Replaces the two parallel scripts (fakewallet.sh, regtest.sh) with one
modules/dev-env/scripts/dev.sh — `dev up [--fakewallet|--regtest]`,
plus `down|logs|shell`. Default mode is fakewallet (no docker, no
chains, instant), matching what the prior scaffold did with two scripts
but giving consumers one command and one verb-set to learn.

Drops the now-redundant `lnbits.backend` enum and `features.fakewallet`
option from core.nix. Backend selection is the dev CLI's runtime
concern; a NixOS-level option would be a second knob that can disagree
with the CLI flag at runtime. `lnbits.{host,port}` stay (bind addr,
useful to docs and any later service path). `features.regtest` stays
(gates docker engine installation — consumers who'll never use the
regtest mode shouldn't pay for the container engine).

Strips modules/lnbits.nix entirely. The dev CLI runs lnbits ad-hoc; if
a NixOS-managed lnbits service becomes a real ask later, re-add a
focused module then.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

configuration.nix

@@ -24,13 +24,10 @@
     # Option schema (lnbits-sensei.*).
     ./modules/core.nix
 
-    # LNbits service wrapper.
-    ./modules/lnbits.nix
-
     # Git remote topology — upstream / fork / extras.
     ./modules/git/remotes.nix
 
-    # Dev environment (worktree mgmt, regtest, fakewallet, tmux).
+    # Dev environment (worktree mgmt, regtest stack, dev CLI, tmux).
     ./modules/dev-env
   ];